1. Field of the Present Invention
The method of the present invention rejuvenates solid paraxylene adsorbent, often referred to as an adsorbent bed, improving output yield of the monomer. More particularly, the method proposes flushing increased levels of water, with a polar organic compound therein, if desired, through aged adsorbent in an adsorption separator to effectively increase functionality of the adsorbent in removing paraxylene from a mixed monomer stream.
2. Description of Prior Art
Starting in the early 1970's, the use of solid adsorbents to extract paraxylene from mixtures of paraxylene, metaxylene, orthoxylene and ethylbenzene was introduced, with most presently existing plants using the adsorption technology. In such systems, the adsorbent is held stationary in liquid filled chambers and external liquid phase streams are manipulated to simulate a moving bed process. Feeds are derived from the fractionated (and sometimes extracted) high severity reforming of virgin naphthas. Desorbents include toluene, mixed diethylbenzene and paradiethylbenzene.
It has further been taught that continuous injection of a constant small amount of water is required to maintain the selectivity of the adsorbent with typical injection rates centering around 200 wt PPM of the liquids flowing to the adsorbent. In fact, changing the injection rate of water is claimed to have a deleterious effect on selectivity and recovery. Thus, according to the prior art, the optimum water injection rate is a constant 200 wt PPM. An exemplary teaching may be found in the Yan U.S. Pat. No. 4,705,909.
Further, such solid adsorbents typically have required replacement upon aging, the adsorbent losing effectivity over a period of 5 to 10 years of use.
To replace the adsorbent, the production of paraxylene must be stopped for about two months. First, the hydrocarbons must be removed using procedures that require at least one week of elapsed time. Second, the used adsorbent must be unloaded, a three to four week effort. Usually, the internals in the adsorbent chambers are also removed and repaired, further lengthening the down time. Finally, expensive new adsorbent and the repaired internals are loaded into the adsorbent chambers. Because of the costs, it can easily require several years to recover the combination of lost production for the two month period and the cost of the new adsorbent.
Thus, it will be understood that development of a method for extending the useful life of adsorbent by rejuvenating same would be crucial to production of paraxylene.